Rotate device to portrait orientation (causing an artifact including a portion of the red and green boxes to be displayed)
As a bit of explanation, revealing of the green box causes the contents of the “block_wrapper” div to overflow. This triggers the “overflow-y: auto” CSS. Thus, the first requirement to replicate the problem is the presence of “overflow-y: auto” on the container and an overflowing of the container in the vertical direction (causing the content in the container to be scrollable in the vertical direction).
The second requirement is that content within the body (e.g., the blue content in the “background_content” div) overflow the viewport in the vertical direction (causing the content within the body to be scrollable in the vertical direction).
I refer to the remnants of the red and green boxes as an “artifact” since you cannot interact with them. In other words, while the red and green boxes are scrollable in the landscape orientation after step 4, they are not scrollable in the portrait orientation after step 6. Also, if text is present behind the artifact (as it is in the actual site I am working on), it can be selected through the artifact while the artifact remains and covers the selected text.
This issue occurs on a Lumia 640 LTE (AT&T) running Windows 10 Mobile (OS build 10.0.10586.218) and also running Windows Phone 8.1 Update 2 (OS version: 8.10.15148.160), and maybe others. It does not occur on any other device running other operating systems (e.g., Android, iOS, Kindle Fire, etc.).

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